Plural frequency musical instrument oscillator



June 13, 1967 R. B. SCHRECONGOST 3,325,747

PLURAL FREQUENCY MUSICAL INSTRUMENT OSCILLATOR Filed April 14, 1966 United States Patent Delaware Filed Apr. 14, 1966, Ser. No. 542,626 15 Claims. (Cl. 331-60) The present invention relates to electrical musical instruments, electric organs for instance, primarily as concerned with the generation of musical tone signals.

In such instruments there must be present in some form a source for each tone frequency the instrument is called upon to play. In the past many schemes have been devised for accomplishing this including the use of individual electronic oscillators, one for each note, rotating tone wheels, photoelectric scanning systems and so on.

In general, this invention is directed to an oscillator acting as a tone generator that can deliver more than one frequency of useful tones. Specifically, by combining the outputs of a pair of interconnectedamplifiers which share a parallel resonant circuit for double frequency output and the output from one amplifier for single frequency, applicant provides a dual frequency output oscillator.

Among many objects of the invention are the followmg:

To provide a novel oscillator which is adapted to provide two frequencies simultaneously, one twice the other;

To provide a novel oscillator having low cost and a minimum number of components;

To provide a novel oscillator having a tank circuit in the input which can be frequency controlled over a small musical range by altering a bias supplied to the input;

To provide a novel oscillator adapted to produce two output frequencies an octave apart with each output frequency having a full range of harmonics; and

To provide a novel oscillator having a dual frequency output which is frequency modulated by an input signal to achieve a vibrato effect for each output frequency.

Other objects and advantages will become apparent sistances R54, R50 and R52 and leads 30 and 32, respectively, to bases of transistors 20 and 22, respectively. The collector of transistor 20 is connected by lead 38, resistor R1, capacitor C7 to the base of transistor 22. Similarly, the collector of transistor 22 is connected by lead 40, resistor R2, capacitor C to the base of transistor 20.

Between the junction of R1 and C7 and R2 and C5 is a tank circuit composed of inductance L1 and tuningcapacitors C1 and C3. The junction of the capacitors C1 and C3 is grounded.

from the following description of a preferred embodiment of the invention which is illustrated in the accompanying drawings. In the drawings in which similar characters of reference refer to similar elements throughout the several views:

FIG. 1 is a schematic diagram showing an example of the prior art;

FIG. 2 is a schematic diagram illustrating one embodiment of the invention;

FIGS. 3a, b. c, and d are wave voltages at points ofFIG. 2;

FIG. 4 is a modification FIG. 2;

FIGS. 5:: and 5b are wave form diagrams showing the output voltages of FIG. 4; and

FIG. 6 is a further modification of the invention shown in FIG. 2..

In FIG. 1 is seen a frequency source of the prior art. It is an essentially conventional emitter feedback Hartley circuit. There is a single frequency output at 48.

of the invention shown in form diagrams of the A parallel resonant circuit T1 is connected at one end to the input of a transistor 10 and the emitter feeds a sig nal back to an intermediate point on the inductor of the tank circuit. Terminal 24 provides a forward bias and low frequency vibrato potential.

Referring to FIG. 2 which illustrates a basic embodiment of applicants invention, it consists as shown of a pair of NPN transistors 20 and 22 connected as follows. The forward bias and low frequency vibrato signal is supplied to a terminal 24 which is connected through re- The emitters are connected together by lead 54-and grounded through resistor R56. The collector of transistor 20 is connected by resistor R36, lead 34, and resistor R58 to a positive supply voltage terminal 60. The collector of transistor 22 is similarly connected through resistor R42 to lead 34 and resistor R58 to the positive supply terminal 60.

An input to the oscillator is provided through terminal 24 supplying a vibrato and bias signal through resistor R54 and resistors R50 and R52 to the base of transistors 20 and 22 respectively. An output signal is provided from the collector of transistor 22 through capacitor C9 to terminal 48. This output signal is at the LC tank frequency. A second output frequency, double the first output frequency, is taken from lead 34 which is at the junctionof resistors R36 and R42 in the collector circuits of the transistors 20 and 22, respectively, through ca pacitor C13 and grounded resistor R13 at terminal 50.

FIG. 2 illustrates the simplest version of the two fre: quency oscillator. It uses a common untapped. coil L1 and a common emitter restor R56. The tank circuit formed of inductor L1 and capacitors C1, C3 is shared by the transistors. The transistor bases require some resistive isolation R50, R52 relative to each other, but the common resistor R54 is much larger and keeps substantially the. same voltages supplied to the transistor bases. The col lector resistors R36 and R42 are brought together into a common resistor R58 to receive pulses from each tran-j sistor as each contributes current in its half cycle.

In sustaining oscillation, when the tank voltage becomes positive, for example to the base of transistor 20, its collector supplies feedback energy to the tank by means of the resistance R1 until limiting occurs. The feedback signals are so phased that the respective collector signals reinforce the tank voltage excursions. Relatively high loading on the tank by the feedback resistors is tolerable because of the fact that a low Q tank enhances the effect of a seven cycle vibrato voltage injected on the base bias at terminal 24.

The operation of the circuit of FIG. 2 will now be discussed with respect to FIG. 3 which discloses wave forms at various points in the circuit of FIG. 2. FIG. 3a shows the voltage wave form at the double frequency output 50, and FIG. 3b shows the voltage wave form at the single frequency output point 48. FIG. 30 discloses the voltage wave form at the collector of transistor 20. This voltage is fed back through resistor R1 and capacitor C7 to the base of transistor 22. FIG. 3d discloses a double frequency voltage at a lower signal level for the voltage through capacitor C11 at point d which is connected to the emit ters by line 54.

To avoid some subharmonics it is necessary to balance the complementary resistors and capacitors in the circuit. At the frequency of operation P1 of 350 to 700 cycles in the present design the second frequency range F2 is 700 to 1400 cycles. In this range subharmonics are easily heard and the collector outputs should be balanced to keep the subharmonics to less than 1.5%.

Because the transistors alternately saturate and cut off, they are of no particular concern in matching. Neither the bias R50, R52 or feed back R1, R2 resistors nor the tuning capacitors C1 or C3 are significant contributors to subharmonic content. The collector resistors R36 and R42 contribute alternate signals to R58 and should be matched to each other to within 1%. The capacitor C13 can be of considerable help here if carefully chosen to offer a higher impedance to the subharmonic frequency than to the higher frequency 2F.

Representative component specifications used for the circuit of FIG. 2 are as follows: C1 mfd .047 C3 mfd .047 C5 mfd .047 C7 rrrfd .047 C9 mfd .01 C11 mfd .01 C13 mfd .0047 L1 h F1 c.p.s.. 350-700 Transistors 2N2926 R1 ohms 47K R2 ohms 47K R36 ohms 10K R42 ohms 10K R50 ohms 47K R52 ohms 47K R54 ohms 220K R56 ohms 150 R58 ohms 4700K As noted above with respect to FIG. 3b, this wave form contains an additional small pulse between t2 and :3 which contributes to the even harmonics of the lower frequency wave form somewhat, but matches favorably with other full harmonic wave forms used in organ design.

Loading of the organ circuit at this point 48 must be kept low so as not to unduly unbalance the double frequency signal and induce subharmonics. The output is high, however, and large isolation resistance can be used. The embodiment of FIG. 4 eliminates the necessity for careful balancing as the transistor collector signal is fed independently to the double frequency output terminal 50, and because the transistor saturation is very predictable, balance is assured. The extra small pulse at the output from terminal 48 is also avoided.

FIG. 4 is thus a modification of the inventive circuit of FIG. 2. It differs from the circuit of FIG. 2 in having a pair of diodes D2, D4 connected to the transistor collectors. These diodes are connected to a DC bias terminal 26 through resistor R60 so as to pass a positive voltage to the collector of transistors 20 and 22. The double frequency output is taken from the junction of these diodes through capacitor C15 and grounded resistor R15 to terminal 50. The single frequency output is taken as in FIG. 2 through the collector of transistor 22 and capacitor C9 at terminal 48 which is isolated from terminal 50. Resistors R36 and R42 are in the collector circuits of transistors and 22 as in FIG. 2. However, the positive voltage supply from terminal 60 is connected directly to line 34.

FIG. 5a illustrates the double frequency output from terminal 50, and FIG. 5b shows the single frequency output from terminal 48. It should be noted that there is no extra pulse in the output from terminal 48 as in the circuit of FIG. 2 as seen in FIG. 3b t2-t3.

The novelty of this circuit may be seen in the unique method of connecting two transistors to one tank using a resistor feedback from opposite collectors and using either an untapped inductor or tapped inductor. The simplicity in the use of an untapped inductor is preferable from cost considerations. The use of the diodes in combining the alternate collector pulses into a double frequency signal is noteworthy. However, the double frequency signal may also be taken across the common emitter resistor R56. This circuit can deliver a balanced sine wave across the tank because of the alternate half cycle driving function. Also, this circuit can be frequency controlled, for example for vibrato, over a small musical range by altering the bias 24 supplied to the bases.

In FIG. 6 is disclosed an alternative tapped coil embodiment of the dual output oscillator of this invention. This circuit is essentially two oscillators using a single tapped tank circuit. FIG. 6 differs from the embodiment of FIG. 2

- in having a tapped inductor L2 in a parallel resonant circuit. Resistors R62 and R64 connect the transistor emitters to points along the tapped coil.

The operation of this embodiment is similar to FIG. 2. A symmetrical sine wave is obtained across the tank as in the embodiment of FIG. 4.

Various modifications may be made in the invention without departing from the spirit and scope thereof and it is desired therefore that only such limitations shall be placed thereon as are imposed by the prior art and are set forth in the appended claims.

I claim:

1. A dual output musical instrument oscillator comprising a first and second amplifier each amplifier having an input and output circuit interconnected, the improvement comprising a tank circuit connected between the input of each amplifier, a bias signal input connected to each of said amplifier inputs, an impedance connected to each of said amplifier output circuits said impedances connected together and to a DC supply at their other terminal whereby a first output signal is obtained from the ouput of the first amplifier and a second output signal is obtained from the junction of said impedances whose frequency is twice the first output signal frequency.

2. In a dual output musical instrument oscillator, thecombination recited in claim 1 wherein the first and second output signals have a full range of harmonics.

3. In a dual output musical instrument oscillator, the combination recited in claim 2 wherein said oscillator is a sine wave oscillator.

4. In a dual output musical instrument oscillator, the combination recited in claim 2 wherein said bias signal includes a vibrato signal to frequency modulate the output signals for vibrato effect.

5. In a musical instrument oscillator comprising a first and second transistor each having an input and output interconnected, the improvement comprising a tank circuit connected between the inputs of each transistor, a

bias signal input connected to said transistor inputs, a pair of diodes reversely poled in a direction to pass unidirectional current to the output circuits of each of the transistors whereby a first output signal is obtained from said first transistor and a second output signal is obtained from the junction of the diodes whose frequency is twice the first output signal frequency.

6. An oscillator comprising a first and second transistor each transistor having an input and output interconnected, the improvement comprising a tank circuit connected between the input of each transistor, a'bias signal input connected to the transistor inputs, an impedance in each of the transistor output circuits connected together and to a DC supply whereby a'first output signal is obtained from the output of said first transistor and a second output signal is obtained from the junction of said impedances whose frequency is twice the first output signal frequency.

-7. In a dual output musical instrument oscillator, the combination recited in claim 6 wherein the first and second output signals have a full range of harmonics.

8. In an oscillator the combination recited in claim 7 wherein said bias signal includes a low cycle AC to cause a vibrato in each output signal.

9. A musical instrument oscillator comprising a first and second transistor each transistor having an input and ouput interconnected, the improvement comprising a tank circuit connected between the bases of each transistor, an impedance in each of the transistor collector circuits said impedances connected in series, a DC supply connected to said impedances at their junction whereby a first all harmonic output signal is obtained from the collector of said first transistor and a second all harmonic output signal is obtained from the junction of said collector transistor impedances whose frequency is twice the first output signal frequency.

10. In a musical instrument oscillator, the combination recited in claim 9 wherein said tank circuit comprises an untapped inductor.

11. In a musical instrument oscillator, the combination recited in claim 9 wherein a bias means is provided for each of said transistor inputs and a low cycle AC signal supplied to said bias means to frequency-shift each output signal.

12. An oscillator comprising a first and second transistor each having an input and output interconnected, the improvement comprising a tank circuit including the parallel connection of an inductor and a pair of capacitors in series connected to the input of each transistor, a bias signal input connected to said transistor inputs, a pair of diodes said diodes being reversely poled in the direction of the output circuit of each of the transistors and adapted to pass unidirectional current to the output circuit of each transistor whereby a first output signal is obtained from said first transistor and a second output signal whose frequency is twice the first output signal frequency is obtained from the junction of said diodes.

13. A dual output musical instrument oscillator comprising a first and second amplifier each amplifier having an input and output circuit interconnected, the improvement comprising a tank circuit connected between the input of each amplifier, an impedance connected to each of said amplifier output circuits said impedances connected together and to a DC supply at their other terminal whereby a first output signal is obtained from the output of the first amplifier and a second output signal is obtained from the junction of said impedances whose frequency is octavely related to the first output signal frequency.

14-. In a musical instrument oscillator, the combination recited in claim 13 having signal means connected to each of said amplifier inputs to vary the frequency of the first and second output signals.

15. In a musical instrument oscillator, the combination recited in claim 14 wherein said signal means supplies a cyclic voltage.

No references cited.

ROY LAKE, Primary Examiner. I. KOMINSKI, Assistant Examiner. 

1. A DUAL OUTPUT MUSICAL INSTRUMENT OSCILLATOR COMPRISING A FIRST AND SECOND AMPLIFIER EACH AMPLIFIER HAVING AN INPUT AND OUTPUT CIRCUIT INTERCONNECTED, THE IMPROVEMENT COMPRISING A TANK CIRCUIT CONNECTED BETWEEN THE INPUT OF EACH AMPLIFIER, A BIAS SIGNAL INPUT CONNECTED TO EACH OF SAID AMPLIFIER INPUTS, AN IMPEDANCE CONNECTED TO EACH OF SAID AMPLIFIER OUTPUT CIRCUITS SAID IMPEDANCES CONNECTED TOGETHER AND TO A DC SUPPLY AT THEIR OTHER TERMINAL WHEREBY A FIRST OUTPUT SIGNAL IS OBTAINED FROM THE OUTPUT OF THE FIRST AMPLIFIER AND A SECOND OUTPUT SIGNAL IS OBTAINED FROM THE JUNCTION OF SAID IMPEDANCES WHOSE FREQUENCY IS TWICE THE FIRST OUTPUT SIGNAL FREQUENCY. 